A digital radio frequency (RF) modulator provides modulation for base-band TV signals. The RF modulator provides direct conversion of digital base-band audio and video signals to a desired RF channel frequency, without any analog up conversion. The RF modulator includes an audio module, a video module, and a RF converter. The audio module includes a pre-emphasis filter, a multi-stage audio interpolator and a complex frequency modulator to generate frequency modulated (FM) audio signals. The video module includes a complex VSB filter, a group-delay compensation filter and some processing logic to generate a filtered output video signal. The RF converter includes a complex adder, a complex multiplier and a RF interpolator to construct the base band TV signals and to shift the base band TV signals in a frequency domain to the desired RF channel frequency. The exponential video carrier is generated at baseband and has a frequency whose value is in the range of +/−13.5 MHz. The RF interpolator includes a zero pad logic followed by a quadrature band pass filter (BPF), and an optional second stage of another zero-pad logic followed by a real band pass filter (BPF). The second stage is optional in the sense that it is required only if the desired RF channel is in the higher VHF band.
Legal claims defining the scope of protection, as filed with the USPTO.
1. A digital radio frequency (RF) modulator for modulating a base-band TV signal with a RF channel carrier frequency, said digital RF modulator comprising: an audio modulator configured to receive input audio signals and generate frequency modulated (FM) audio carrier; a video modulator configured to receive a composite video base-band signal (CVBS) and generate a filtered output video signal; and a RF converter connected to said audio modulator and said video modulator, said RF converter comprising: a complex adder configured to add the frequency modulated (FM) audio signals and the filtered output video signal to form a first sampling rate base-band TV signal having a first plurality of spectrum images; a complex frequency shifter configured to receive the first sampling rate base-band TV signal from an output of said complex adder and multiply the first sampling rate base-band TV signal with an exponential carrier having a frequency selected from a range of frequencies so as to frequency shift one of the plurality of spectrum images of the first sampling rate base-band TV signal uni-directionally by the selected frequency of the exponential carrier to a location at the RF channel carrier frequency; and a RF interpolator configured to receive the frequency shifted selected spectrum image of the first sampling rate base-band TV signal from said complex frequency shifter and increase said first sampling rate by a factor of N>1 to a second sampling rate.
2. The digital modulator as claimed in claim 1 further comprising an audio interface configured to provide the input audio signals to said audio modulator.
3. The digital modulator as claimed in claim 1 further comprising a video interface configured to provide the composite video base-band signal (CVBS) to said video modulator.
4. The digital modulator as claimed in claim 1 further comprising an output interface configured to provide the modulated base-band TV signal to a DAC.
5. The digital modulator as claimed in claim 1 further comprising a processing interface for configuring the digital RF modulator to communicate with one or more of a general purpose processor and a memory module.
6. The digital modulator as claimed in claim 1 , said audio modulator comprising: a pre-emphasis filter configured to amplify high frequency input audio signals and output pre-emphasized audio signals; a multi stage audio interpolator configured to increase an audio sampling rate of said pre-emphasized audio signals and generate interpolated audio samples; and a complex frequency modulator configured to frequency modulate the interpolated audio samples with an exponential carrier.
7. The digital modulator as claimed in claim 1 , said video modulator comprising: a group delay compensation filter configured to pre-correct for non linear phase characteristics of a receiver; an IRE level adjustor configured for scaling the composite video base-band signal CVBS with pre-calculated constant values; and a complex VSB filter configured to pass only one complete sideband and a vestige of another sideband.
8. The digital modulator as claimed in claim 7 , wherein said VSB filter has a programmable bandwidth dependent on a targeted television system.
9. The digital modulator as claimed in claim 8 , wherein said targeted television system is one of a National Television Standards Committee (NTSC) system, a phase alternation line (PAL) system, and a SECAM system.
10. The digital modulator as claimed in claim 1 , said RF interpolator producing a second plurality of spectrum images of said base-band TV signal, said RF converter further comprising: a complex bandpass filter centered at the RF channel carrier frequency and configured to select one of the second plurality of spectrum images centered at the RF channel carrier frequency.
11. The digital modulator as claimed in claim 10 , wherein said range of frequencies of the exponential carrier lies in a range of +/−said first sampling rate/2 MHz.
12. The digital modulator as claimed in claim 1 , wherein said RF interpolator is operable to increase said first sampling rate by the factor of N>1 to the second sampling rate comprises by inserting N−1 logical zeroes between two consecutive samples of the frequency shifted selected spectrum image of the first sampling rate base-band TV signal.
13. The digital modulator as claimed in claim 1 , wherein said RF interpolator is further configured to increase said second sampling rate base-band TV signal by a factor of M>1 to a third sampling rate.
14. The digital modulator as claimed in claim 13 , wherein said RF interpolator is operable to increase said second sampling rate by the factor of M>1 to the third sampling rate comprises inserting M−1 logical zeroes between two consecutive samples of the frequency shifted selected spectrum image of the second sampling rate base-band TV signal.
15. The digital modulator as claimed in claim 13 said increasing producing a third plurality of spectrum images of said base-band TV signal, said RF interpolator further comprising a bandpass filter operable to select one of the third plurality of spectrum images centered at another RF channel carrier frequency.
16. A method of modulating base-band TV signals to an RF channel carrier frequency, said method comprising: processing input audio signals to generate frequency modulated (FM) audio signals through an audio modulator; processing input composite video base-band (CVBS) signals to generate a filtered output video signal through a video modulator; adding the frequency modulated (FM) audio signals and the filtered output video signals to form a first sampling rate base-band TV signal having a first plurality of spectrum images; shifting one of the first plurality of spectrum images of the first sampling rate base-band TV signal in a frequency domain to a location of the RF channel carrier frequency by multiplying with an exponential carrier having a frequency selected from a range of frequencies; and increasing said first sampling rate by a factor of N>1 to a second sampling rate through an interpolation of the frequency shifted selected spectrum image of the first sampling rate base-band TV signal.
17. The method as claimed in claim 16 , said processing the input audio signals comprising: amplifying high frequency input audio signals through a pre-emphasis filter; increasing audio sampling rate of pre-emphasized audio signals through a multi stage audio interpolator; and modulating the frequency of an exponential carrier with the interpolated audio samples through a complex frequency modulator.
18. The method as claimed in claim 16 , said processing the input composite video base-band (CVBS) signals comprising: sampling the input CVBS signals to twice a pixel rate through an interpolator if the input CVBS samples are at the pixel rate; pre-correcting non linear phase characteristics of a receiver through a group delay compensation filter; adjusting the amplitude of the CVBS signal through an IRE level adjustor; and generating a VSB spectrum through a complex VSB filter.
19. The method as claimed in claim 16 , said increasing producing a second plurality of spectrum images of said base-band TV signal, said method further comprising bandpass filtering to select one of the second plurality of spectrum images centered at the RF channel carrier frequency.
20. The method as claimed in claim 16 , wherein increasing said first sampling rate by the factor of N>1 to the second sampling rate comprises inserting N−1 logical zeroes between two consecutive samples of the frequency shifted selected spectrum image of the first sampling rate base-band TV signal.
21. The method as claimed in claim 16 , further comprising increasing said second sampling rate by a factor of M>1 to a third sampling rate through an interpolation of the frequency shifted selected spectrum image of the second sampling rate base-band TV signal.
22. The method as claimed in claim 21 , wherein increasing said second sampling rate by the factor of M>1 to the third sampling rate comprises inserting M−1 logical zeroes between two consecutive samples of the frequency shifted selected spectrum image of the second sampling rate base-band TV signal.
23. The method as claimed in claim 21 , said increasing producing a third plurality of spectrum images of said base-band TV signal, said method further comprising bandpass filtering to select one of the third plurality of spectrum images centered at another RF channel carrier frequency.
24. The method as claimed in claim 16 , wherein said range of frequencies of the exponential carrier lies in a range of +/−said first sampling rate/2.
25. A processing system for processing a frequency modulated (FM) audio carrier and a composite video base-band signal (CVBS), comprising an addition process operable to add the frequency modulated (FM) audio signal and a filtered version of the composite video base-band signal to form a first sampling rate base-band TV signal having a first plurality of spectrum images; a complex frequency shifting process operable to multiply the first sampling rate base-band TV signal with an exponential carrier having a frequency selected from a range of frequencies so as to frequency shift one of the plurality of spectrum images of the first sampling rate base-band TV signal uni-directionally by the selected frequency of the exponential carrier to a location at a first RF channel frequency; and an RF interpolation process operable to increase said first sampling rate of the frequency shifted selected spectrum image of the first sampling rate base-band TV signal by a factor of N>1 to a second sampling rate.
26. The processing system of claim 25 , wherein said range of frequencies of the exponential carrier lies in a range of +/−said first sampling rate/2.
27. The processing system of claim 25 , said RF interpolator process producing a second plurality of spectrum images of said base-band TV signal, and further comprising a complex bandpass filtering process operable to select one of the second plurality of spectrum images centered at the first RF channel frequency.
28. The processing system of claim 25 , wherein said RF interpolation process is configured to insert N−1 logical zeroes between two consecutive samples of the frequency shifted selected spectrum image of the first sampling rate base-band TV signal.
29. The processing system of claim 25 , further comprising an additional RF interpolation process operable to increase said second sampling rate base-band TV signal by a factor of M>1 to a third sampling rate.
30. The processing system of claim 29 , wherein said RF interpolation process is configured to insert M−1 logical zeroes between two consecutive samples of the frequency shifted selected spectrum image of the second sampling rate base-band TV signal.
31. The processing system of claim 29 , said increasing producing a third plurality of spectrum images of said base-band TV signal, and further comprising a bandpass filtering process operable to select one of the third plurality of spectrum images centered at a second RF channel frequency.
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August 20, 2007
May 22, 2012
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